Molecular controlled semiconductor device

ABSTRACT

A semiconductor sensing device for sensing presence, absence or level of species-of-interest in the environment is disclosed. The semiconductor sensing device comprises at least one layer of molecules deposited thereon. The molecules are electrically-responsive to the species-of-interest in a manner such that when the molecules interact with the species-of-interest, a reverse breakdown voltage characterizing the semiconductor sensing device is modified.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.11/292,370, filed on Dec. 2, 2005, which claims benefit of U.S.Provisional Patent Application No. 60/632,536, filed on Dec. 3, 2004.The contents of the above Applications are all incorporated herein byreference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device and, moreparticularly, to a molecular controlled semiconductor sensing device.

Semiconducting materials are well known and include n-type and p-typesemiconductors, so named because either they have an excess of electrons(“−”; n-type semiconductor) or a deficit of electrons than what isnecessary to complete a lattice structure (“+”; p-type semiconductor).The extra electrons in the n-type material and the holes (deficit ofelectrons) left in the p-type material serve as negative and positivecharge carriers, respectively. Semiconductor devices typically includeat least one p-n junction, which is a border region between an n-typeand p-type semiconductor. The p-n junction possesses properties, whichcan be used in many electronic applications, such as diodes,transistors, memory media and the like.

A diode is an electronic device that allows current flow (i.e.,electronic conduction) in one direction but prevents current flow (i.e.,is insulating) in the opposite direction. Commonly, the conductive andinsulating states of a diode are referred to as a “forward bias” and“reverse bias” effects, respectively, where the term “bias” correspondsto the application of electric voltage to the p-n junction.

In forward bias, the holes in the p-type region and the electrons in then-type region are pushed towards the p-n junction. Application of aforward bias is by connecting the p-type region to a positive terminalof a voltage source and the n-type part to a negative terminal of thevoltage source. With such voltage configuration, the positive chargeapplied to the p-type region repels the holes, while the negativecharge, applied to the n-type region, repels the electrons. This reducesthe junction barrier, allowing the electrons to overcome this barrierand enter the p-type region. Once inside the p-type region, theelectrons make their way to the positive terminal of the power supply,hence generating an electric current.

In reverse bias, the p-type region is connected to the negative terminaland the n-type region is connected to the positive terminal of the powersupply, thus pulling the holes in the p-type and the electrons in then-type away from the p-n junction. This effectively widens the p-njunction, increasing the electrical resistance to the flow of electrons.Up to a certain voltage, commonly referred to as the breakdown voltage,regular diodes practically prevent current flow therethrough. Byexceeding the breakdown voltage, a regular diode is destroyed due toexcess current and overheating.

A Zener diode is a diode device especially designed to have a greatlyreduced breakdown voltage, also known as the Zener voltage, named afterC. M. Zener (1905-1993). A Zener diode contains a heavily doped p-njunction allowing electrons to tunnel from the valence band of thep-type material to the conduction band of the n-type material. Acurrent-voltage curve characterizing the Zener diode comprises a regionof forward current at a forward voltage (about 0.7 volts for silicondiode), a region of reverse or breakdown current at the Zener voltageand a flat region of small (practically zero) current therebetween.Thus, upon application of a reverse bias, the Zener diode exhibits acontrolled breakdown and lets current flow in a manner such that thevoltage across the Zener diode is kept at the Zener voltage. Thebreakdown voltage of a Zener diode can be accurately controlled in thedoping process of the semiconductor materials forming the p-n junction.

An avalanche diode is another diode device designed to provide abreakdown current. It this device, the breakdown is by impact ionizationrather than by the Zener effect. When no or small reverse voltage isapplied to the avalanche diode, thermal energy results in formations ofa few electron-hole pairs in the depletion region of the p-n junction.When a sufficient reverse voltage (i.e., above the breakdown voltage) isapplied across the p-n junction, the electrons accelerate in theelectric field, collide with the atoms of the semiconductor lattice, andrupture their covalent bonds to form more pairs. The released electronsalso accelerate in the electric field, resulting in a chain or avalancheeffect of carrier multiplication in which further electron-hole pairsare released. The avalanche effect releases an almost unlimited numberof carriers so that the avalanche diode essentially becomes a shortcircuit. The current flow in this region is limited only by an externalseries current-limiting resistor. Once the reverse voltage is removed,all the charge carriers return to their normal energy values andmomenta.

A Schottky diode is a diode device which, unlike the p-n junction of aconventional diode, has a metal-semiconductor junction in which the workfunction of the metal and the band gap of the semiconductor are selectedto reduce the voltage across the junction. Such junction is termedSchottky barrier, after Walter H. Schottky (1886-1976).

A field effect transistor (FET) is a semiconductor device having asource electrode, a drain electrode, a gate electrode and a channel,which is separated from the gate electrode by a thin isolating layer.The channel has semiconducting properties (either n-type or p-typesemiconducting properties) such that the density of charge carrierstherein can be varied. When no voltage is applied to the gate electrode,the channel does not contain any free charge carriers and is essentiallyan insulator. Upon application of a certain level of voltage to the gateelectrode, an electric field, generated between the channel and thegate, attracts charge carriers (electrons or holes) from the sourceelectrode and the drain electrode, and the channel becomes conductive.

Semiconductor materials and devices can be used as transducers insensing applications whereby semiconductor materials are combined with asensing element responsive to chemicals or energy. For example, in U.S.Pat. No. 4,777,019, the contents of which are hereby incorporated byreference, small monomers of macromolecules are directly introduced intothe surface layer of a semiconductor, to thereby form a biosensor havingan improved signal to noise ratio. The use of semiconductor materials astransducers in sensing devices is an attractive option because sensingdevices employing other transducers (e.g., electrochemical,piezoelectric or optical transducers) suffer from various limitations,including high cost, complexity and/or bulkiness. Contrarily, thecombination of semiconductors with sensing molecules enjoys theselectivity, sensitivity and versatility of molecular synthesis as wellas the benefits of robustness and proven technology of today'soptoelectronics (to this end see, e.g., “Physics, Chemistry, andTechnology of Solid State Gas Sensor Devices”, by A. Mandelis and C.Christofides, Wiley, New York, 1993).

Generally, the design of sensing devices is aimed at achievingsensitivity, selectivity, robustness and versatility. However, combiningthese qualities in the process of designing an electronic sensor wasproven very difficult and many of today's sensors are optically innature, requiring a detector system to couple to electronic circuits.The use of molecules (e.g., organic molecules) as sensing elements inelectronic detectors has the benefit of versatility and selectivity, butis not associated with robustness, especially when organic molecules areemployed, because in many cases electrons are required to flow throughthe organic medium, thus causing the destruction of the sensing element.In other cases, e.g., when no organic medium is employed, or when noelectron flow through the organic medium takes place, sensitivitybecomes the limiting parameter. Generally, sensitivity of sensors isproportional to the contact area of the sensitive surface, because thelarger the area the higher the probability that a molecule or photon canbe detected by the sensing surface. Thus, in general, sensitivity isassumed to scale with surface area.

In Molecular Controlled Semiconductor Resistors (MOCSER), the moleculesare adsorbed directly on the surface of a semiconductor device [U.S.Pat. No. 6,433,356, Gartsman, K. et al., Chem. Phys. Lett., 283:301-306,1998]. This is in contrast to most other devices, where the chemicalsare adsorbed on the gate metal or insulator layer of a metal oxide FET(MOSFET), or on the surface metal of a Schottky diode. These deviceshave limiting sensitivity because of the insulating film of the MOSFETand/or the metal.

The combination between molecules and semiconductor devices is alsorelevant for biological systems, as it allows use of moleculesirrespective of their ability to form good monolayers, their electricalconductivity or their stability against electron transport through them[Ashkenasy et al., Acc. Chem. Res., 35, 121-128, 2002]. The ability toaffect electronic properties of a semiconductor surface by adsorption oflayers of (organic) molecules has been demonstrated and used to achieveselectivity.

Apart from the MOCSER several molecular semiconductor sensors are knownin the art. Chemically modified FET (CHEMFET) sensors are based onchanges in the current passing through the device due to adsorption ofmolecules on the gate. Un-gated (open gate FET; OGFET) sensors orsurface accessible FET (SAFET) sensors include molecules, which areadsorbed on the surface normally covered by the gate metal between thesource and the drain. These types of sensors, however, suffer fromover-sensitivity to electrical interference due to their open gatestructure, leading to high noise levels compared to devices withchannels completely covered by a gate oxide and/or metal.

Most prior art FET sensors use MOSFET-like structures because therelatively low barrier height that characterizes silicon devices leadsto high leakage currents. An intrinsic problem one faces withMOSFET-like structures is that the oxidation layer on the surfacereduces the sensitivity to adsorbed chemicals. This problem can beovercome by using molecular layers for both noise reduction (surfacestabilization) and gating the FET, as disclosed in U.S. Pat. No.6,433,356 and Gartsman et al. supra. Best results were obtained byconstructing MOCSERs from special GaAs/(Al,Ga)As structures, which aresimilar to high electron mobility transistors, but without a gateelectrode. Apart from the cost, use of GaAs and related materials leadsto problems for in vivo use of such sensors and to ease of incorporationin present day Si-based electronic technologies.

There is thus a widely recognized need for, and it would be highlyadvantageous to have a molecular controlled electronic sensor, devoid ofthe above limitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided asemiconductor sensing device. The semiconductor sensing devicecomprises: (a) a device body made of at least two regions ofsemiconductor material forming at least one p-n junction thereamongst,wherein charge carrier concentrations of the at least two regions ofsemiconductor material are selected such that a current-voltagecharacteristic of the at least one p-n junction comprises apredetermined reverse breakdown voltage; and (b) at least one layer ofmolecules deposited on at least one of the at least two regions of thesemiconductor material, the molecules being electrically-responsive to aspecies-of-interest in a manner such that when the molecules interactwith the species-of-interest, the predetermined reverse breakdownvoltage is modified.

According to further features in preferred embodiments of the inventiondescribed below, the regions of semiconductor material comprise a firstregion, a second region and a third region, the first region being madeof a first type semiconductor material and the second and third regionsbeing made of a second type semiconductor material.

According to still further features in the described preferredembodiments the second region is disposed on or formed in the firstregion so as to at least partially interpose between the first regionand the third region.

According to still further features in the described preferredembodiments the semiconductor sensing device further comprises anadditional electrode for electrically controlling the predeterminedreverse breakdown voltage.

According to still further features in the described preferredembodiments the additional electrode at least partially engages asurface of the device body.

According to still further features in the described preferredembodiments the additional electrode is at least partially buried in thedevice body.

According to still further features in the described preferredembodiments the additional electrode comprises a perforated electrodedisposed on the third region and being connectable to a voltage source.

According to still further features in the described preferredembodiments the additional electrode comprises a buried structure havinga semiconductor electrode connectable to a voltage source and asemiconductor barrier, wherein the buried structure is formed in thedevice body in a manner such that the semiconductor electrode and thedevice body are interposed by the semiconductor barrier.

According to still further features in the described preferredembodiments the molecules are deposited on the third region.

According to still further features in the described preferredembodiments the semiconductor sensing device further comprises at leasttwo conducting pads for connecting the device to a voltage source,wherein each conducting pad is formed in, attached to, or integratedwith one region of the at least two regions of semiconductor material

According to still further features in the described preferredembodiments the semiconductor sensing device further comprises acovering film deposited on at least one of the at least two regions.

According to another aspect of the present invention there is provided amethod of manufacturing a semiconductor sensing device. The methodcomprises: (a) providing a device body and forming therein at least tworegions of semiconductor material, so as to define at least one p-njunction, wherein charge carrier concentrations of the at least tworegions of semiconductor material are selected such that acurrent-voltage characteristic of the at least one p-n junctioncomprises a predetermined reverse breakdown voltage; and (c) depositingat least one layer of molecules on at least one of the at least tworegions of the semiconductor material, the molecules beingelectrically-responsive to a species-of-interest in a manner such thatwhen the molecules interact with the species-of-interest, thepredetermined reverse breakdown voltage is modified.

According to further features in preferred embodiments of the inventiondescribed below, the method further comprises depositing a covering filmon at least one of the at least two regions.

According to still further features in the described preferredembodiments the method further comprises forming integrating orattaching the conducting pads to the at least two regions.

According to still further features in the described preferredembodiments the method further comprises forming an additional electrodein the device body for electrically controlling the predeterminedreverse breakdown voltage.

According to still further features in the described preferredembodiments the formation of the additional electrode comprisesdepositing a perforated electrode on the third region in a manner suchthat the perforated electrode is connectable to a voltage source.

According to still further features in the described preferredembodiments the formation of the additional electrode comprises buryingin the device body a semiconductor electrode and a semiconductor barrierin a manner such that the semiconductor electrode is connectable to avoltage source and the semiconductor barrier interposes between thesemiconductor electrode and the device body.

According to yet another aspect of the present invention there isprovided a method of sensing presence, absence or level ofspecies-of-interest in the environment, the method comprising: (a)providing semiconductor sensing device having at least one layer ofmolecules deposited thereon, wherein the molecules areelectrically-responsive to the species-of-interest in a manner such thatwhen the molecules interact with the species-of-interest, a reversebreakdown voltage characterizing the semiconductor sensing device ismodified; (b) applying a reverse bias to the semiconductor sensingdevice; (c) exposing the semiconductor sensing device to theenvironment; and (d) using the modifications in the reverse breakdownvoltage for sensing presence, absence or level of thespecies-of-interest.

According to further features in preferred embodiments of the inventiondescribed below, the method further comprises placing an additionalsemiconductor sensing device in an isolated environment and comparing areverse breakdown voltage of the additional semiconductor sensing devicewith the reverse breakdown voltage of the semiconductor sensing device.

According to still further features in the described preferredembodiments, the sensing of presence, absence or level of thespecies-of-interest comprises scanning a voltage of the applied reversebias.

According to still further features in the described preferredembodiments the sensing of presence, absence or level of thespecies-of-interest comprises determining presence or absence of reversecurrent in a fixed voltage of the applied reverse bias.

According to still further features in the described preferredembodiments the interaction of the molecules with thespecies-of-interest comprises absorption of the species-of-interest bythe molecules.

According to still further features in the described preferredembodiments the method further comprising heating the semiconductorsensing device so as to desorb the species-of-interest off themolecules.

According to still further features in the described preferredembodiments the semiconductor sensing device comprises a device bodymade of at least two regions of semiconductor material forming at leastone p-n junction thereamongst, wherein charge carrier concentrations ofthe at least two regions of semiconductor material are selected suchthat a current-voltage characteristic of the at least one p-n junctioncomprises a predetermined reverse breakdown voltage.

According to still further features in the described preferredembodiments the molecules and the charge carrier concentrations areselected such that the modification of the reverse breakdown voltage isaccompanied by generation of an avalanche current through the at leastone p-n junction.

According to still further features in the described preferredembodiments the electrical response of the molecules is characterized inthat a charge of the molecules and a respective region of the at leasttwo regions is modified when the molecules interact with thespecies-of-interest.

According to still further features in the described preferredembodiments the electrical response of the molecules is characterized inthat a dipole moment of the molecules and a respective region of the atleast two regions is modified when the molecules interact with thespecies-of-interest.

According to still further features in the described preferredembodiments the molecules and the charge carrier concentration of thethird region are selected such that a combination of the molecules andthe third region is characterized by a predetermined dipole moment, thepredetermined dipole moment being modified when the molecules interactwith the species-of-interest.

According to still further features in the described preferredembodiments the charge carrier concentration of the first region of thefirst type semiconductor material is larger than the charge carrierconcentration of the second and the third regions of the second typesemiconductor material.

According to still further features in the described preferredembodiments a thickness of the second region is at least three times acharacteristic Debye length thereof. According to still further featuresin the described preferred embodiments a thickness of the third regionis from about two times to about five times a characteristic Debyelength thereof.

According to still further features in the described preferredembodiments the second region describes a closed shape surrounding thethird region of the second type semiconductor material. According tostill further features in the described preferred embodiments the closedshape has a width of at least two times the characteristic Debye lengthof the second region. According to still further features in thedescribed preferred embodiments the closed shape has an inner diameterof at least five times the characteristic Debye length of the secondregion.

The present invention successfully addresses the shortcomings of thepresently known configurations by providing a molecular controlledelectronic sensor that enjoys properties far exceeding the prior art.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods andmaterials are described below. In case of conflict, the patentspecification, including definitions, will control. In addition, thematerials, methods, and examples are illustrative only and not intendedto be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

In the drawings:

FIGS. 1 a-b are schematic illustrations of a side view (FIG. 1 a) and atop view (FIG. 1 b) of a semiconductor sensing device, according tovarious exemplary embodiments of the present invention;

FIG. 2 is a schematic illustration of the semiconductor sensing devicein a preferred embodiment in which a buried structure is employed;

FIG. 3 is a flowchart diagram describing a method of sensing presence,absence or level of species-of-interest in the environment, according tovarious exemplary embodiments of the present invention;

FIG. 4 is a flowchart diagram describing method of manufacturing asemiconductor sensing device, according to various exemplary embodimentsof the present invention; and

FIG. 5 shows the dependence of the reverse breakdown voltage on thecharge carrier of one semiconductor region of the device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a molecular controlled semiconductor sensingdevice which can be used for sensitive and selective sensing of avariety of species-of-interests, such as, but not limited to, photons,chemical substances, biological materials and the like. Specifically,the present invention can be used to sense the species-of-interest bydetecting modifications in a breakdown voltage characterizing thedevice.

The principles and operation of a semiconductor sensing device accordingto the present invention may be better understood with reference to thedrawings and accompanying descriptions.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

While conceiving the present invention it was hypothesized and whilereducing the present invention to practice it was realized that sensingof species-of-interest can be achieved using modifications of reversebreakdown voltage. Specifically, it was uncovered by the Inventors ofthe present invention that the characteristic reverse breakdown voltageof a semiconductor device varies when molecules deposited on thesemiconductor device interact with the species-of-interest. Thus,according to one aspect of the present invention there is provided asemiconductor sensing device, referred to hereinafter as device 10.

Referring now to the drawings, FIGS. 1 a-b illustrate a side view (FIG.1 a) and a top view (FIG. 1 b) of device 10, according to variousexemplary embodiments of the present invention. In its simplestconfiguration, device 10 comprises a semiconductor device body 12 andone or more layers of molecules 20 which are electrically-responsive toa species-of-interest 22. Preferably, molecules 20 form a partial orcomplete monolayer on the surface of device 10. The electric response ofmolecules 20 is realized using the modification (increment or decrement)of a reverse breakdown voltage of device 10 upon interaction event withthe species 22, either directly (voltage) or via the change in currentat a given voltage.

Device body 12 is made of two or more regions of semiconductor materialforming at least one p-n junction 24 thereamongst. In the exemplaryconfiguration shown in FIGS. 1 a-b, device 10 comprises a first region14, a second region 16 and a third region 18. It is to be understoodthat other configurations having a different arrangement and/or adifferent number of regions are not excluded from the scope of thepresent invention.

According to various exemplary embodiments of the present invention,molecules 20 are deposited (e.g., adsorbed) on surface 19 of thirdregion 18. Molecules 20 are preferably bifunctional in a sense thatmolecules 20 are capable of both chemically binding to surface 19 andinteracting with species 22, e.g., by absorption or formation of achemical bond. Representative examples of a chemical bond that can beestablished between molecules 20 and surface 19 or species 22 include,without limitation, a covalent bond, an electrostatic bond, a coordinatebond, a hydrogen bond and a van der Waals bond. Representative examplesof molecules 20 include, without limitation, various chloro- andalkoxysilanes, such as 2,3-aminopropyltrimethoxy-silane (APTS); variouscarboxylic acids, such as tartaric, malonic and succinic acid, as wellas their derivatives (e.g., 2,3-di(p-cyanobenzoyl) tartaric acid);various alkenes, phoponates, phosphates, thiols, sulfides and theirderivatives (e.g., 4,5-di(p-cyano-benzoyloxy)-1,2-dithiane).

Molecules 20 and the surface to which they bind form a structure havingelectrostatic properties (e.g., charge concentration, dipole momentetc.), which preferably remain substantially constant as long asmolecules 20 do not interact with the species-of-interest. Oncemolecules absorb photons or bind to another molecule, the electrostaticproperties are modified resulting in a new electrostatic configurationof device 10 and, consequently, a modification of the reverse breakdownvoltage.

As further exemplified in the example section that follows, the variousregions of device 10 can be constructed such that the current-voltagecharacteristics of the p-n junction(s) comprise a predetermined reversebreakdown voltage. Specifically, by a judicious selection of the chargecarrier concentrations of the semiconductor materials, a reversebreakdown voltage ranging from about 2 volts to about 20 volts can beobtained.

As used herein the term “about” refers to ±10%.

Sensing device 10 can operate in more than one way. In one preferredembodiment, device 10 operates similarly to a Zener diode, in which casethe onset or cessation of a reverse current through device 10 is used todetermine presence or absence of the species-of-interest. This operationresembles the operation of a Zener diode, which is typically used inelectrical circuits as a dynamical on/off switch for the purpose ofvoltage stabilization.

Thus, according to the presently preferred embodiment of the invention,a reversebias is applied to device 10 to maintain a voltage drop acrossdevice 10. Preferably, the voltage drop is slightly above or slightlybelow the breakdown voltage, depending on the nature of the modificationof the reverse breakdown voltage caused by the interactions between thedeposited molecules and the species-of-interest.

For example, if the interaction increases the breakdown voltage, theexistence of a reverse current can correspond to absence of thespecies-of-interest and the cessation of the reverse current cancorrespond to a detection event. This can be achieved by applying thereverse bias above the breakdown voltage, to allow a reverse current toflow through device 10. The level of applied reverse bias is preferablyselected such that upon an interaction event, the breakdown voltage isincreased to beyond the applied bias. Thus, in this embodiment, aninteraction event results in a cessation of the reverse current, becausethe voltage drop on device 10 is no longer sufficient to allowbreakdown.

Similarly, if the interaction between the molecules and the speciesdecreases the breakdown voltage, the onset of the reverse current cancorrespond to a detection event. This can be achieved by applying thereverse bias below the breakdown voltage, such that upon an interactionevent, the breakdown voltage is decreased to below the applied bias,resulting in an onset of reverse current through device 10.

In another embodiment, device 10 preferably has a time-dependentresponse, hence operates similarly to a diode in which the breakdownmechanism is due to the avalanche effect. Thus, according to thepresently preferred embodiment of the invention, the molecules, thecharge carrier concentrations and/or the thickness of the depletionregion of the p-n junction(s) are selected such that the modification ofthe reverse breakdown voltage is accompanied by generation of anavalanche current through the p-n junction. This can be achieved byselecting a sufficiently wide p-n junction so as to allow the chargecarriers to collide with lattice atoms. In this embodiment, themolecules are preferably selected such that their interaction decreasesthe breakdown voltage. In operation, a reverse bias, slightly below thebreakdown voltage, is applied to device 10. Once an interaction eventoccurs, the breakdown voltage decreases to below the applied reversebias and charge multiplication is triggered. The resulting avalanchecurrent is proportional to the number of interactions and device 10 canbe used as a differential sensor, which is sensitive both to thepresence and to the level (e.g., number, concentration) of thespecies-of-interest.

In an additional, yet preferred, embodiment, the device is exposed tothe species-of-interest, and the applied reverse bias is scanned over apredetermined voltage range so as to determine whether or not thereverse breakdown voltage is modified. In this embodiment the devicedoes not have to be subjected to a reverse bias during the exposure. Thescan over the voltage range can be done subsequently to the exposure. Itis to be understood that scanning of the voltage contemporaneously withthe exposure is also contemplated.

The semiconductor regions of device 10 can be made of any semiconductor,such as, but not limited to, those that have as their mainconstituent(s): Silicon, group IV alloys, or combinations of the III andV elements, the so-called III-V compounds, where the groups III, IV andV denote the Periodic Table elements: III=B, Al, Ga, In or Tl; IV=Si, Geor C; and V=N, P, As, Sb or Bi. For example, according to variousexemplary embodiments of the present invention, regions 14, 16 and 18are made of crystalline silicon, whereby region 14 has p-typeconductivity, and regions 16 and 18 have n-type conductivity.Alternatively, region 14 can have n-type conductivity, while regions 16and 18 have p-type conductivity. In any event, the charge carrierconcentrations of the regions, as stated, are selected to obtain thedesired reverse breakdown voltage of device 10. The charge carrierconcentrations of regions 14, 16 and 18 are denoted hereinunder by N₁,N₂ and N₃, respectively.

Second region 16 is preferably disposed on or formed in first region 14so as to at least partially interpose between regions 14 and 18. Thus,according to various exemplary embodiments of the present inventionregion 16 describes a closed shape surrounding region 18. For example,region 16 can be shaped as a circular ring, a square collar and thelike. The advantage of this configuration is that the ring-like shape ofthe region 16 substantially reduces edge effects resulting in asubstantial reduction or elimination of the sensitivity of device 10 tosurface conductivity. The thickness, width and inner diameter of region16 are preferably substantially larger than its characteristic Debyelength.

A Debye length can be defined as the distance in a semiconductor overwhich the local electric field affects the distribution of free chargecarriers. The Debye length, L, is inversely proportional to the squareroot of the charge carrier concentration, N, and can be calculated fromthe following formula (in SI units):

$\begin{matrix}{{{L_{i}\left( N_{i} \right)} = \sqrt{\frac{4{\pi ɛɛ}_{0}{kT}}{q^{2}N_{i}}}},} & (1)\end{matrix}$

where, ∈₀ is the dielectric permittivity of vacuum, ∈ is the dielectricconstant of the semiconductor q is the elementary charge, k is theBoltzmann constant and T the is temperature. The subscript i in Equation1 above denotes the number of the semiconductor region. Thus, in thepresent embodiment, L₁ L₂ and L₃ correspond to the Debye lengths offirst region 14, second region 16 and third region 18, respectively.

According to a preferred embodiment of the present invention, In anyevent, the dimensions of the semiconductor regions of device 10 arepreferably selected so as to improve the signal to noise ratio. Thus,depending on the specific application for which device 10 is designed,the thickness of region 16 is preferably larger or equals 2L₂, its widthis from about 2L₂ to about 8L₂ and its inner diameter larger or equals5L₂. The thickness of region 18 is preferably from about 2L₃ to about5L₃. It is to be understood that the above dimensions are not to beconsidered as limiting.

It was found by the inventors of the present invention that an optimalconfiguration can be achieved when the diameter of region 18 is about 1micron.

Additionally, N₂, the charge carrier concentration of region 16 ispreferably lower than both N₁ and N₃, the charge carrier concentrationsof regions 14 and 18. For example, in various exemplary embodiments ofthe invention N₁ is selected to be on order of 10¹⁸ or more, e.g., fromabout 10¹⁸ to about 3×10¹⁹ cm⁻³, N₂ is about two orders of magnitudesmaller than N₁, e.g., from about 1×10¹⁶ cm⁻³ to about 5×10¹⁷ cm⁻³ andN₃ is on the same order of magnitude as N₂ however larger, e.g., N₃≧3N₂.According to a preferred embodiment of the present invention the valueof N₃ is selected in accordance with the desired level of breakdownreverse current. Specifically, lower doping of region 18 corresponds toa larger extent of the avalanche effect, hence higher attainable levelof breakdown reverse current.

Referring now again to FIGS. 1 a-b, device 10 preferably comprises atleast two conducting pads 26 for connecting device 10 to a voltagesource (not shown). Pads 26 can be formed in, attached to or integratedwith the semiconductor regions of device 10. For example, in oneembodiment, pads 26 are metal contacts prepared on the surfaces ofregions 16 and 14, such that when pads 26 are connected to the voltagesource, a reverse bias is established across junction 24. According tovarious exemplary embodiments of the present invention, device 10further comprises a covering film 28 deposited on at least one of theregions. Preferable, molecules 20 remain exposed to allow them tointeract freely with the species-of-interest. Thus, according to apreferred embodiment of the present invention film 28 covers regions 14and 16 and exposes region 18. Film 28 can be any covering film known inthe art, such as, but not limited to, silicon oxide, silicon nitridepassivation layer and/or photoresist or similar polymer.

As stated, in various exemplary embodiments of the invention, device 10is subjected to a reverse bias, either slightly above or slightly belowthe reverse breakdown voltage, depending on the nature and level of themodification to the breakdown voltage caused in response to theinteraction with the species-of-interest. The reverse bias is preferablyestablished between the n-type semiconductor regions (e.g., regions 16and 18) and the p-type semiconductor regions (e.g., region 14).

According to a preferred embodiment of the present invention thecharacteristic reverse breakdown voltage of device 10 can also becontrolled electrically. This embodiment is particularly useful when theextent of the breakdown voltage modification caused by certainspecies-of-interest and/or different types of molecules 20 isinsufficient to generate detection event. The present embodimenttherefore further improves the sensitivity of device 10 by allowing finetuning of the characteristic reverse breakdown voltage.

The aforementioned electrical control can be achieved by providing oneor more additional electrodes which, once connected to a voltage source,tune the characteristic reverse breakdown voltage of device 10 todesired level. This can be done in more than one way. For example, inone embodiment, the additional electrode preferably comprises aperforated electrode 30, disposed on third region 18. A voltage sourceis thus connected to pads 26 and perforated electrode 30. Thecharacteristic reverse breakdown can be tuned by varying the potentialof electrode 30.

Reference is now made to FIG. 2, which is a schematic illustration ofdevice 10, according to another preferred embodiment of the presentinvention. Hence, in this embodiment, device 10 comprises a buriedstructure 32 formed in device body 12 having a semiconductor electrode34 and a semiconductor barrier 36. Buried structure 32 serves forelectrically controlling the characteristic reverse breakdown of device10.

Both electrode 34 and barrier 36 of structure 32 are preferably of thesame type semiconductor, preferably the opposite type of first region14. According to a preferred embodiment of the present invention barrier36 has a very low charge carrier concentration (e.g., N<5×10¹⁶ cm⁻³) andelectrode 34 is more heavily doped (e.g., N>5×10¹⁷). Thus, in thisembodiment, barrier 36 serves as an insolating barrier between region 14and electrode 36. The voltage source is thus connected to pads 26 andelectrode 36, whereby the characteristic reverse breakdown is tuned byvarying the potential of electrode 36. As will be appreciated by oneordinarily skilled in the art, the applied voltage affects the chargecarrier concentration of a portion 38 of region 14, which is adjacent toregion 18, thereby allowing the fine-tuning of the reverse breakdownvoltage.

According to another aspect of the present invention, there is provideda method of sensing presence, absence or level of species-of-interest inthe environment. The method comprises the following method steps, whichare illustrated in the flowchart diagram of FIG. 3. It is to beunderstood, that unless otherwise defined, the method steps describedhereinbelow can be executed either contemporaneously or sequentially inany combination or order of execution. Specifically, neither theordering of the flowchart of FIG. 3, nor the numerals designating itsvarious blocks are to be considered as limiting. For example, two ormore method steps, appearing in the description or in the flowchart ofFIG. 3 in a particular order, can be executed in a different order(e.g., a reverse order) or substantially contemporaneously.

Referring now to FIG. 3, in step 42 of the method a semiconductorsensing device (e.g., device 10) is provided and in step 44 a reversebias is applied to the sensing device, as further detailed hereinabove.In step 46 of the method, the sensing device is exposed to theenvironment so as to allow the species (if present) to interact with thesensing element of the device (e.g., the aforementioned bifunctionalmolecules). The method continues to step 48 in which modifications inthe reverse breakdown voltage of the device are used to sense thespecies-of-interest. The sensing can be done in more than one way. Inone preferred embodiment, the voltage of the applied reverse bias isscanned so as to determine the breakdown voltage of the device, hencethe presence, absence or level of the species-of-interest. In anotherembodiment, the reverse breakdown voltage modifications are monitored bydetermining presence or absence of reverse current while keeping theapplied voltage fixed.

According to a preferred embodiment of the present invention the methodcomprises an additional step (step 50 in FIG. 3), in which two or moredevices are used so as to decrease thermal drift of the measurements.Hence, according to the presently preferred embodiment of the inventionan additional semiconductor sensing device (e.g., device 10) is placedin an isolated environment. The reverse breakdown voltage of theadditional device is preferably compared with the reverse breakdownvoltage of the exposed sensing device, for example, using an electricalbridge scheme.

In embodiments in which the sensing device is used to sensespecies-of-interest which are absorbed or bind by the sensing element ofthe device, the method preferably continues to step 52, in which thesensing element of the device is heated so as to desorb thespecies-of-interest. This embodiment is particularly useful when it isdesired to reuse the device. The desorption of the species restores theoriginal reverse breakdown voltage of the device, thus facilitating itsreuse. The heating can be done by any way known in the art. A preferredheating procedure includes the exploitation of the breakdown current forheating, for example, by reducing the electrical resistance of theelectrical circuit to which the device is connected.

According to an additional aspect of the present invention, there isprovided a method of manufacturing a semiconductor sensing device. Themethod comprises the following method steps, which are illustrated inthe flowchart diagram of FIG. 4. Similarly to FIG. 3 above, the methodsteps described hereinbelow can be executed either contemporaneously orsequentially in any combination or order of execution.

Hence, according to a preferred embodiment of the present invention themethod starts at step 60 in which at least two regions of semiconductormaterial are formed in a device body. The charge carrier concentrationsof formed regions are selected, as stated, to provide predeterminedreverse breakdown voltage. The regions can be formed in the device bodyusing any standard photolithographic and micro-machining technique knownin the art.

In step 62, one or more layers of molecules which are electricallyresponsive to the species-of-interest are deposited on one or more ofthe semiconductor regions of the device as further detailed hereinabove.The method preferably continues to step 64 in which a covering film isdeposited on selected regions of the device (e.g., regions 14 and 16).Additional steps of the method include, without limitations, theconnection of conducting pads (step 66) and the formation of theperforated electrode and/or buried structure (step 68) as furtherdetailed hereinabove.

Additional objects, advantages and novel features of the presentinvention will become apparent to one ordinarily skilled in the art uponexamination of the following examples, which are not intended to belimiting. Additionally, each of the various embodiments and aspects ofthe present invention as delineated hereinabove and as claimed in theclaims section below finds experimental support in the followingexample.

EXAMPLES

Reference is now made to the following examples, which, together withthe above descriptions, illustrate the invention in a non limitingfashion.

Example 1 Theoretical Considerations Current-Voltage Characteristics inthe Absence of the Species-of-Interest

The voltage-current curve of device 10 before it is exposed to thespecies-of-interest is determined by the three contributions:

1. A surface leakage current.

2. A “generation” current of the p-n junction:

$\begin{matrix}{{I_{g} \approx {{{qn}_{i}^{2}\left\lbrack {{A_{21}\left( {\frac{D_{1}}{\lambda_{1}N_{1}} + \frac{D_{2}}{L_{2}N_{2}}} \right)} + {A_{31}\left( {\frac{D_{1}}{\lambda_{1}N_{1}} + \frac{D_{3}}{\lambda_{3}N_{3}}} \right)}} \right\rbrack}\left( {A/{cm}^{2}} \right)}},} & (2)\end{matrix}$

where A₁₂ is the area between the first and second regions, A₁₃ is thearea between the first and third regions, D_(i) (i=1, 2, 3) is thediffusion coefficient of charged carries in the corresponding regions,n, is the intrinsic carrier concentration of the semiconductor, andλ_(i) is the carrier diffusion length in the corresponding regions.

3. Tunneling current, mainly between the first and third regions, givenby:

$\begin{matrix}{{J \approx {q\sqrt{\frac{kT}{2\pi \; m_{1}}}{N_{1} \cdot \left( A_{13} \right) \cdot {\exp \begin{pmatrix}{{- \frac{4}{3}}\frac{\sqrt{2m}}{q\; \hslash}\frac{E_{g}^{3/2}}{V_{a}}} \\\sqrt{\frac{q}{2{{ɛɛ}_{0}\left( {\varphi_{bi} + V_{a}} \right)}}\frac{N_{1} \cdot N_{3}}{N_{1} + N_{3}}}\end{pmatrix}}}\left( {A/{cm}^{2}} \right)}},} & (3)\end{matrix}$

where E_(g) is the band gap of the semiconductor, h is Planck constant,V_(a) is the applied voltage, φ_(bi) is the built-in voltage dropbetween the first and third regions. The voltage drop φ_(bi) is givenby:

$\begin{matrix}{\varphi_{bi} \approx {\frac{q}{kT}{{\ln \left( \frac{N_{1}N_{3}}{n_{i}^{2}} \right)}.}}} & (4)\end{matrix}$

Typically, when the applied bias is below the reverse breakdown voltage,the sum of the surface, generation and tunneling currents does notexceed a few tens of μA/cm².

On the other hand, if the applied bias exceeds a breakdown voltage, thecurrent increases sharply and is limited only by a ballast resistorwhich protects the measuring circuit. The breakdown voltage, V_(b), canbe estimated as:

$\begin{matrix}\begin{matrix}{V_{b} \approx {E_{b}\sqrt{\frac{q}{2{{ɛɛ}_{0}\left( {\varphi_{bi} + V_{a}} \right)}}\frac{N_{1} \cdot N_{3}}{N_{1} + N_{3}}}}} \\{\approx {E_{b} \cdot \sqrt{\frac{{qN}_{3}}{2{{ɛɛ}_{0}\left( {{\frac{q}{kT}{\ln \left( \frac{N_{1}N_{3}}{n_{i}^{2}} \right)}} + V_{a}} \right)}}}}}\end{matrix} & (5)\end{matrix}$

where V_(a) is the voltage level of the applied bias and E_(b) is thebreakdown electric field of the semiconductor. Typically, E_(b) is aboutseveral 100 kV/cm and weakly depends on the concentration of the chargecarriers.

As can be understood from Equation 5, the strong dependence of thereverse breakdown voltage on N₃ allows to accurately set the reversebreakdown voltage by selecting an appropriate value for N₃.

FIG. 5 shows the dependence of the reverse breakdown voltage on N₃ for asilicon based device with N₁=10¹⁹ cm⁻³.

Effect of the Species of Interest on the Current-Voltage Characteristic

As stated, the species-of-interest can change the dipole moment of thestructure formed by the molecules and the surface to which they bind.Many species-of-interest have a permanent dipole moment. Therefore,their interaction with the molecules affects the dipole moment of themolecules-surface structure. Adsorption of the species onto the moleculethus changes the electrical potential at the top surface of the thirdregion and, thereby, also modifies the distribution of the chargedcarriers.

The effect of the molecules can be viewed as attraction or repulsion ofthe charged carriers to the surface of the third region. When the widthof the third region is only a few Debye lengths, the interaction of thespecies-of-interest with the molecules-surface structure changes theconcentration of the charged carriers near the p-n junction interposingthe first and third regions, leading to a change of the breakdownvoltage.

The sensitivity of the device, in volts per monolayer of absorbedmolecules, can be understood as follows. A monolayer of small organicabsorbed molecules corresponds to a surface density, S_(m), of about10¹⁴ molecules/cm⁻². The ability of the molecules to attract or to repelthe electrons towards the surface is a function of the total dipolemoment of the molecule-surface structure. A typically value of theattraction or to repulsion ability, c, is larger than or equal to about10⁻³ electrons per molecule.

A monolayer covering f=1% of the surface thus leads to theredistribution of ≈10¹⁰ electrons/cm². The total concentration ofelectrons per unit area in the third region is given by d₃·N₃≈10¹¹−10¹³cm⁻². Therefore, 1% of a monolayer causes a change of about 0.1-10% inthe electron concentration in the third region. As will be appreciatedby one ordinarily skilled in the art, such a change is sufficient tomodify the reverse breakdown voltage.

The responsivity, R, in volts per monolayer is given by

$R = {\frac{\partial V_{r}}{\partial N_{3}}\frac{{cS}_{m}}{d_{3}}}$

and is in the range of 0.3 to 30 volts per monolayer, depending on thedoping level and the thickness of the third region. For example, forN₃=10¹⁷ cm⁻³ and d₃=80 nm the responsivity is about 180 millivolts permonolayer. The sensitivity of the device is defined by the accuracy withwhich the breakdown voltage can be measured. Presently availablemeasuring devices can measure a breakdown voltage as low as 100microvolts, corresponding to a sensitivity of about 0.001% of amonolayer. The accuracy of the measurements of the breakdown voltagedepends on the electrical noise in the device. The electrical noise, inits turn, depends on the resistances of regions 16 and 18. Therefore,the final dimensions of the semiconductor regions depend on the specificapplications for which the device is designed.

Example 2 Prototype Device

A device was prepared in accordance with FIGS. 1 a-b. The device had asensitivity of about 1 volts per monolayer with respect to succinicacid.

First region 14 was a p-type semiconductor and second 16 and third 18regions were each n-type semiconductor. The charge carrierconcentrations regions 14, 16 and 18 were, respectively: N₁=10¹⁹ cm⁻³,N₂=5×10¹⁶ cm⁻³ and N₃=5×10¹⁷ cm⁻³. The area of third region 18 was 4μm². Molecules 20 were prepared by treating the SiO_(x) (1.5 nm) on Sisurface with 3-aminopropyltrimethoxy-silane solution for 20 min at roomtemperature.

Exposure of the device to 0.2 mM solution of succinic acid caused ashift of the breakdown voltage from 4.65 volts to 5.40 volts. Assumingcomplete coverage, one can deduce that the sensitivity of the device wasclose to 1 volt per monolayer.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1. A semiconductor sensing device, comprising: (a) a device body made ofat least two regions of semiconductor material forming at least one p-njunction thereamongst, wherein charge carrier concentrations of said atleast two regions of semiconductor material are selected such that acurrent-voltage characteristic of said at least one p-n junctioncomprises a predetermined reverse breakdown voltage; and (b) at leastone layer of molecules deposited on at least one of said at least tworegions of said semiconductor material, said molecules beingelectrically-responsive to a species-of-interest being at least one of aphoton, a chemical substance and a biological material in a manner suchthat upon exposure of the device to said species-of-interest, saidmolecules interact with said species-of-interest, and said predeterminedreverse breakdown voltage is modified.
 2. The device of claim 1, whereinsaid molecules and said charge carrier concentrations are selected suchthat said modification of said reverse breakdown voltage is accompaniedby generation of an avalanche current through said at least one p-njunction.
 3. The device of claim 1, wherein said electrical response ofsaid molecules is characterized in that a charge of said molecules and arespective region of said at least two regions is modified when saidmolecules interact with said species-of-interest.
 4. The device of claim1, wherein said electrical response of said molecules is characterizedin that a dipole moment of said molecules and a respective region ofsaid at least two regions is modified when said molecules interact withsaid species-of-interest.
 5. The device of claim 1, further comprising acovering film deposited on at least one of said at least two regions. 6.The device of claim 1, further comprising at least two conducting padsfor connecting the device to a voltage source, wherein each conductingpad of said at least two conducting pads is formed, attached orintegrated with one region of said at least two regions of semiconductormaterial.
 7. The device of claim 1, further comprising an additionalelectrode for electrically controlling said predetermined reversebreakdown voltage.
 8. The device of claim 7, wherein said additionalelectrode at least partially engages a surface of said device body. 9.The device of claim 7, wherein said additional electrode is at leastpartially buried in said device body.
 10. The device of claim 1, whereinsaid at least two regions of semiconductor material comprise, a firstregion a second region and a third region, said first region being madeof a first type semiconductor material and said second and said thirdregions being made of a second type semiconductor material.
 11. Thedevice of claim 1, wherein said second region is disposed on or formedin said first region so as to at least partially interpose between saidfirst region and said third region.
 12. The device of claim 10, furthercomprising an additional electrode for electrically controlling saidpredetermined reverse breakdown voltage.
 13. The device of claim 12,wherein said additional electrode comprises a perforated electrodedisposed on said third region and being connectable to a voltage source.14. The device of claim 12, wherein said additional electrode comprisesa buried structure having a semiconductor electrode connectable to avoltage source and a semiconductor barrier, wherein said buriedstructure is formed in said device body in a manner such that saidsemiconductor electrode and said device body are interposed by saidsemiconductor barrier.
 15. The device of claim 11, wherein saidmolecules are deposited on said third region.
 16. The device of claim15, wherein said molecules and said charge carrier concentration of saidthird region are selected such that a combination of said molecules andsaid third region is characterized by a predetermined dipole moment,said predetermined dipole moment being modified when said moleculesinteract with said species-of-interest.
 17. The device of claim 16,wherein said charge carrier concentration of said first region of saidfirst type semiconductor material is larger than said charge carrierconcentration of said second and said third regions of said second typesemiconductor material.
 18. The device of claim 16, wherein a thicknessof said second region is at least three times a characteristic Debyelength thereof.
 19. The device of claim 16, wherein a thickness of saidthird region is from about two times to about five times acharacteristic Debye length thereof.
 20. The device of claim 18, whereinsaid second region describes a closed shape surrounding said thirdregion of said second type semiconductor material.
 21. The device ofclaim 20, wherein said closed shape has a width of at least two timessaid characteristic Debye length of said second region.
 22. The deviceof claim 20, wherein said closed shape has an inner diameter of at leastfive times said characteristic Debye length of said second region.
 23. Amethod of manufacturing a semiconductor sensing device, the methodcomprising: (a) providing a device body and forming therein at least tworegions of semiconductor material, so as to define at least one p-njunction, wherein charge carrier concentrations of said at least tworegions of semiconductor material are selected such that acurrent-voltage characteristic of said at least one p-n junctioncomprises a predetermined reverse breakdown voltage; and (c) depositingat least one layer of molecules on at least one of said at least tworegion of said semiconductor material, said molecules beingelectrically-responsive to a species-of-interest being at least one of aphoton, a chemical substance and a biological material in a manner suchthat when said molecules interact with said species-of-interest, saidpredetermined reverse breakdown voltage is modified.
 24. The method ofclaim 23, further comprising forming an additional electrode in saiddevice body for electrically controlling said predetermined reversebreakdown voltage.
 25. A method of sensing presence, absence or level ofspecies-of-interest in the environment, the species-of-interest being atleast one of a photon, a chemical substance and a biological material,the method comprising: (a) providing semiconductor sensing device havingat least one layer of molecules deposited thereon, wherein saidmolecules are electrically-responsive to the species-of-interest in amanner such that when said molecules interact with thespecies-of-interest, a reverse breakdown voltage characterizing saidsemiconductor sensing device is modified; (b) applying a reverse bias tosaid semiconductor sensing device; (c) exposing said semiconductorsensing device to the environment; and (d) using modifications in saidreverse breakdown voltage for sensing presence, absence or level of thespecies-of-interest.
 26. The method of claim 25, further comprisingheating said semiconductor sensing device so as to desorb thespecies-of-interest off said molecules.